1. Field of the Invention
The invention pertains to a stable current reference device in integrated circuit form. Devices of this kind are used especially in memory circuits, in particular to generate the stable timing signals needed for the reading or writing of the memory cells.
Current stability is a quality that is desirable for a wide range of temperature on the order of -50.degree. C. to +130.degree. C. Furthermore, it is sought to design circuits capable of working in a range of voltage going from less than two volts up to about five volts. It is therefore necessary to be able to work under low voltage (two volts and less) while at the same time providing for voltage stability in this range. Finally, the variations in characteristics due to the manufacturing method must not have any effect on the reference current so as to obtain high reliability in manufacture.
2. Discussion of the Related Art
It has always been difficult to make current reference devices meeting these criteria of stability, especially in logic technologies such as MOS or CMOS technologies because, in principle, there is no known characteristic of a manufacturing process that can be used to obtain current stability of this kind.
The current reference generation devices known in logic technology are mostly based on the Wilson mirror structure. However, the reference current obtained is fairly dependent on the manufacturing method. There is another type of known device described in the patent application FR 95 09023. This device gives a current based on the difference between the threshold voltage V.sub.tN of an enhanced transistor and a threshold voltage V.sub.tNna of a native transistor having the same type of conductivity. The native transistor drives a reference resistor and the reference current is given by (V.sub.tN -V.sub.tNna)/R. This reference current is stabilized by a negative feedback loop formed by the series connection of a P type MOS transistor and an N type MOS transistor that is a native transistor mounted as a diode on the gate of the native transistor which drives the reference transistor. Nevertheless, the use of a negative feedback to obtain stability is not a very satisfactory approach. Furthermore, in this device, the threshold voltage of the native transistor which drives the reference resistor varies with the source-substrate voltage (substrate effect).
In the invention, another structure in integrated circuit form has been found to provide a stable current reference.
An object of the invention therefore is an intrinsically stable current reference device without negative feedback to compensate for one variation or another.